This invention relates to a stabilised platform arangement which is suitable for use with a body which is movable in a random or largely unpredictable manner, such as is the case with a ship. In such an application, the platform can provide a frame of reference for the boresight direction of an antenna in which the effect of pitch and/or roll of the ship is reduced. If an antenna forms part of a communications network employing satellite repeater stations, it is very important for the bore-sight direction of the antenna to be accurately aligned with the position of the satellite, as the directional beam sensitivity pattern of the satellite or the ship-borne antenna may have a very narrow angle in order to give sufficient signal gain. Pointing errors of the ship-borne antenna can impair or prevent reliable communications. Thus it is necessary for the antenna to be correctly orientated with respect to a distant satellite whilst the satellite itself may traverse an arc of space, even though the ship may pitch and roll in heavy seas.
Stabilisation systems using full servo controlled gyroscopic mechanisms are bulky, complex and expensive, and it has been proposed to mount such an antenna on a platform whose orientations and attitude are stabilised by means of a pendulously mounted gyroscopic assembly. The pendulous nature of the mounting provides a vertical reference line which is actively maintained by a fairly simple gyroscopic assembly which is operative to generate damped restoring forces inhibiting pendulous oscillations. From time to time, however, it can be necessary to rapidly rotate in azimuth the platform on which an antenna is mounted, particularly if an antenna has only limited elevational freedom of movement and is required to continuously track a satellite which passes overhead. There comes a point at which the maximum elevation of the antenna is reached, and an extremely rapid azimuth rotation of the antenna is required if it is to continue correctly tracking the satellite. A rapid azimuth rotation of the gyroscopic assembly can destabilise it, and it may take some considerable time to regain full control of the attitude of the antenna. Uncoupling the gyroscopic assembly from the platform during this period gives rise to difficult mechanical constructions, and makes it difficult for the gyroscopic assembly to properly recommence operation.
The present invention seeks to provide an improved stabilised platform arrangement.